(Newser)
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A new study throws the whole concept of a "due date" into question, finding the length of pregnancy can naturally vary by as much as five weeks. The study of 125 women expands upon what the BBC reports are some fairly broad stats: Only 4% of women actually deliver when predicted, and 30% do not give birth within 10 days of their due dates. As LiveScience explains, that variation was previously attributed to inaccurate calculations of the baby's age. But the US National Institute of Environmental Health Sciences study used urine samples to determine precise ovulation and implantation dates, and, after excluding premature births, found the natural gestation period varied by up to 37 days. (The average ovulation-to-birth time was 268 days.)

The study also found there was a link between gestation length and the age of a mother, the weight she was at birth, and the time it took for embryos to implant. "The emphasis on a single due date may make the length of pregnancy seem more predictable than it really is," says a study researcher. The upshot? Perhaps a "delivered by" date would be better than a "due date," suggests a spokesperson for the UK's Royal College of Obstetricians and Gynecologists.

I recall taking a Statistics course in college and one of the research articles we applied statistical analysis to involved pregnancy length. Of course, I don't recall how to actually apply that analysis all these years later, but I do remember there being a certain number of standard deviations within which pregnancy due dates fall. While 40 weeks was the mean, pregnancies as short as 36 weeks and as long as 44 were completely within the statistically significant range. Just anecdotally, my first pregnancy, had I not been induced, would have gone past the 42-week mark. My second was at exactly 40 weeks. My mother carried my brother for 43 weeks (back when doctors allowed women to carry that long) and my best friend delivered all three of her boys (healthy and fully cooked) at 36 weeks.

Andren_Ess

Aug 8, 2013 10:11 AM CDT

The hospitals are run by accountants and they will tell you, who, when and how. Their spreadsheets depend on a timely delivery, and their scheduling with maximized costs. Not quite on time ! Look for a more pressured consumer experience on the subject. As a poster made an insightful and experienced argument, baby will let you know when they are cooked and ready. Lots of variables effecting both mother and child determine the due date, and to my mind nature knows best. Look forward to a caesarian or some uncomfortable suchers so you stay with the game plan. Prenatal, post natal enter the big money game, but people seem to like this new price on the head for babies. Make sure you sign on the dotted line of course.

bsnodgrass62

Aug 8, 2013 2:33 AM CDT

I have 5 kids and all of them were 3 weeks to a month past due. They were all healthy. I think they are ready when they are ready, and should not rush nature.